In internal combustion engines, oil is typically used for lubrication, cleaning, inhibiting corrosion, to improve sealing, and to cool the engine by carrying heat away from the moving parts. Engine oils are generally derived from petroleum-based and non-petroleum synthesized chemical compounds. Modern engine oils are mainly blended by using base oil composed of hydrocarbons and other chemical additives for a variety of specific applications. Over the course of oil's service life, engine oil frequently becomes contaminated with foreign particles and soluble contaminants, and its chemical properties become degraded due to oxidation and nitration. A common effect of such contamination and degradation is that the oil may lose its capability to fully protect the engine, thus necessitating the used oil to be changed or replaced with clean, new oil.
Engine oil is generally changed based on time in service, or based on a distance the engine's host vehicle has traveled. Actual operating conditions of the vehicle and hours of engine operation are some of the more commonly used factors in deciding when to change the engine oil. Time-based intervals account for shorter trips where fewer miles are driven, while building up more contaminants. During such shorter trips, the oil may often not achieve full operating temperature long enough to burn off condensation, excess fuel, and other contamination that may lead to “sludge”, “varnish”, or other harmful deposits.
To aid with timely oil changes, modern engines often include oil life monitoring systems to estimate the oil's condition based on factors which typically cause degradation, such as engine speed and oil or coolant temperature. When an engine employing an oil life monitoring system is used in a vehicle, such a vehicle's total distance traveled since the last oil change may be an additional factor in deciding on the appropriate time for an oil change.